U.S. patent application number 12/026626 was filed with the patent office on 2011-07-14 for mass spectrometer.
This patent application is currently assigned to SHIMADZU CORPORATION. Invention is credited to Osamu FURUHASHI, Takahiro HARADA, Kiyoshi OGAWA, Koichi TANAKA.
Application Number | 20110168883 12/026626 |
Document ID | / |
Family ID | 39752444 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110168883 |
Kind Code |
A1 |
FURUHASHI; Osamu ; et
al. |
July 14, 2011 |
MASS SPECTROMETER
Abstract
A mass spectrometer is provided that restrains the signal
intensity of an MS/MS spectrum from decreasing according to the
secondary dissociation of a primary fragment ion generated by a
photodissociation. An excitation laser light for causing a
photodissociation is irradiated to the trapping space A in the ion
trap 1. At the same time, an excitation signal that does not excite
a precursor ion but excites fragment ions is applied to the end cap
electrodes 12 and 13. Since the selected precursor ions gather
around the center of the trapping space A, they are irradiated by
the excitation laser light and efficiently dissociated. The
fragment ions generated by this are immediately excited by the
excitation electric field's effect, and are vibrated wildly to be
out of the excitation light irradiated space B. Therefore, the
fragment ions are not easily irradiated by the excitation laser
light and the secondary dissociation does not easily occur.
Inventors: |
FURUHASHI; Osamu; (Uji-shi,
JP) ; HARADA; Takahiro; (Uji-shi, JP) ; OGAWA;
Kiyoshi; (Kizugawa-shi, JP) ; TANAKA; Koichi;
(Kyoto-shi, JP) |
Assignee: |
SHIMADZU CORPORATION
Nakagyo-ku
JP
|
Family ID: |
39752444 |
Appl. No.: |
12/026626 |
Filed: |
February 6, 2008 |
Current U.S.
Class: |
250/288 |
Current CPC
Class: |
H01J 49/0059
20130101 |
Class at
Publication: |
250/288 |
International
Class: |
H01J 49/26 20060101
H01J049/26 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2007 |
JP |
2007-028153 |
Claims
1. A mass spectrometer for mass-analyzing ions generated by a
dissociation in an ion trap, comprising: an ion trap for trapping
ions in a space surrounded by a plurality of electrodes; an
excitation light emitter for irradiating an excitation light for
photo-dissociating ions to a center of a trapping space of the ion
trap; and an excitation signal generator for generating an
excitation signal having a predetermined frequency that does not
make precursor ions to be analyzed resonantly vibrate and
selectively make fragment ions generated by a photodissociation
resonantly vibrate, and for applying the excitation signal to at
least one of the electrodes of the ion trap.
2. The mass spectrometer according to claim 1, further comprising a
gas introducer for introducing a predetermined gas into the ion
trap so as to control a reaction rate of the photodissociation.
3. A mass spectrometer for mass-analyzing ions generated by a
dissociation in an ion trap, comprising: an ion trap for trapping
ions in a space surrounded by a plurality of electrodes; an
excitation light emitter for irradiating an excitation light for
photo-dissociating ions to a space off a center of a trapping space
of the ion trap; and an excitation signal generator for generating
an excitation signal having a predetermined frequency that
selectively makes precursor ions to be analyzed resonantly vibrate
to reach the excitation light irradiated space and does not make
fragment ions generated by a photodissociation resonantly vibrate,
and for applying the excitation signal to at least one of the
electrodes of the ion trap.
4. The mass spectrometer according to claim 3, wherein the
excitation light emitter emits an excitation light to surround a
center of a trapping space of the ion trap.
5. The mass spectrometer according to claim 3, wherein the ion trap
is a three-dimensional quadrupole type ion trap with one ring
electrode and two end cap electrodes, and the excitation light
irradiated area irradiated by the excitation light emitter is away
from a center of the trapping space in the ion trap by 2.5% or
above a distance between the end cap electrodes.
6. The mass spectrometer according to claim 3 wherein the ion trap
is a linear ion trap in which a plurality of rod electrodes with
curved inner surface are aligned parallel, and the excitation light
irradiated area irradiated by the excitation light emitter is away
from a center of the trapping space in the ion trap by 2.5% or
above a distance between inner curved surfaces of two facing rod
electrodes.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a mass spectrometer, and
more specifically to a mass spectrometer using photodissociation
for dissociating ions trapped in an ion trap.
[0002] An MS/MS analysis (or tandem analysis) is a type of
mass-analyzing method. In a typical MS/MS analysis, an ion having a
specific mass is first selected as a precursor ion. Then, the
precursor ion is broken (fragmented) into various product ions (or
fragment ions). Finally, the product ions (or fragment ions) are
subjected to a mass-analyzing process. One of the most widely used
methods for dissociating a precursor ion is a collision-induced
dissociation (CID) process in which a precursor ion is made to
collide with gas atoms or molecules.
[0003] Photodissociation is also one of the methods for
dissociating an ion by irradiating an excitation light onto an ion
to increase its internal energy. Photodissociation includes
ultraviolet light dissociation and Infrared Multiphoton
Dissociation (IRMPD). In ultraviolet light dissociation, an
ultraviolet light as excitation light is irradiated onto an ion.
Then the electronic state of the ion is excited and the
dissociation is accelerated. In IRMPD, an intense infrared light as
excitation light is irradiated onto an ion in order to make the ion
sequentially absorb multiple photons. Then the vibrational state of
the ion is excited and the dissociation is accelerated (See
Non-Patent Document 1 for example). In a mass spectrometer of a
three-dimensional quadrupole type or the like, it is possible to
entrap and hold ions in a comparatively narrow space. Hence, it is
easy to irradiate an excitation light onto one same ion for a
comparatively long period of time. For this reason, an MS/MS
analysis (or an MS' analysis in which dissociations are taking
place in multiple stages) often employs photodissociation (mainly
IRMPD).
[0004] When performing a collision-induced dissociation process
inside of an ion trap, the frequency of a resonant excitation
signal in the ion trap is generally adjusted to the mass of
precursor ions to be analyzed. This selectively excites only the
precursor ions and makes them collide with gas atoms or molecules.
The dissociation (primary dissociation) is accordingly accelerated.
In this case, fragment ions with smaller mass which were produced
by the primary dissociation are not excited so much that they do
not energetically collide with gas atoms or molecules and a
secondary dissociation, or a further dissociation, does not
occur.
[0005] On the other hand, when a photodissociation is performed
inside an ion trap, fragment ions produced by a precursor ion's
dissociation (primary dissociation) by a light absorption are
irradiated with an excitation light together with precursor ions.
Therefore, a secondary dissociation in which a fragment ion is
further photo-dissociated easily occurs. In the case where such a
secondary dissociation takes place, the signal intensity of a
fragment ion (primary fragment ion) produced by a primary
dissociation decreased in an MS/MS spectrum as illustrated in FIG.
9. This results in the severe deterioration of the S/N of a mass
spectrum. [0006] Non-Patent Document 1: L. Sleno et al., "Ion
activation methods for tandem mass spectrometry", Journal of Mass
Spectrometry, 39 (2004), pp. 1091-1112
SUMMARY OF THE INVENTION
[0007] The present invention has been achieved in view of the
aforementioned problems, and a main objective thereof is to provide
a mass spectrometer that enhances the S/N of an MS/MS spectrum by
enhancing the primary dissociation of a precursor ion and
restraining the secondary dissociation as much as possible when a
photodissociation is carried out inside an ion trap for trapping
ions.
[0008] A first aspect of the present invention to solve the
above-described problem provides a mass spectrometer for
mass-analyzing ions generated by a dissociation in an ion trap,
including:
[0009] an ion trap for trapping ions in a space surrounded by a
plurality of electrodes;
[0010] an excitation light emitter for irradiating an excitation
light for photo-dissociating ions to a center of a trapping space
of the ion trap; and
[0011] an excitation signal generator for generating an excitation
signal having a predetermined frequency that does not make
precursor ions to be analyzed resonantly vibrate and selectively
make fragment ions generated by a photodissociation resonantly
vibrate, and for applying the excitation signal to at least one of
the electrodes of the ion trap.
[0012] A second aspect of the present invention to solve the
above-described problem provides a mass spectrometer for
mass-analyzing ions generated by a dissociation in an ion trap,
including:
[0013] an ion trap for trapping ions in a space surrounded by a
plurality of electrodes;
[0014] an excitation light emitter for irradiating an excitation
light for photo-dissociating ions to a space off a center of a
trapping space of the ion trap; and
[0015] an excitation signal generator for generating an excitation
signal having a predetermined frequency that selectively makes
precursor ions to be analyzed resonantly vibrate to reach the
excitation light irradiated space and does not make fragment ions
generated by a photodissociation resonantly vibrate, and for
applying the excitation signal to at least one of the electrodes of
the ion trap.
[0016] In the mass spectrometer according to the first aspect of
the present invention, ions to be analyzed are first trapped as a
precursor ion in the ion trap. The selection of the precursor ions
may be carried out either inside or outside the ion trap. In each
case, the precursor ions concentratedly (i.e. with high
probability) exist around the center of the trapping space of the
ion trap. The excitation light emitter irradiates an excitation
light to the center of the trapping space. At the same time, the
excitation signal generator generates an excitation signal having a
frequency that does not make the precursor ions to be analyzed
resonantly vibrate but make fragment ions with smaller mass
resonantly vibrate. The excitation signal generator applies the
excitation signal to at least one of the electrodes that are
included in the ion trap. For example, in the case where the ion
trap is a three-dimensional quadrupole type ion trap with one ring
electrode and two end cap electrodes, the excitation signal may be
applied between both end cap electrodes because a trapping electric
field is normally formed by applying an RF voltage for trapping
ions to the ring electrode.
[0017] The precursor ions concentratedly existing around the center
of the trapping space are irradiated with the excitation light, and
dissociated by a photodissociation (primary dissociation) to
generate fragment ions. Since the precursor ions are not effected
by the electric field formed by the excitation signal, they do not
vibrate wildly and they effectively receive the excitation light to
be photo-dissociated. On the other hand, since the fragment ions
generated by this are affected by the excitation electric field,
they immediately begin to vibrate wildly and go out of the center
of the trapping space. Accordingly, they are not easily irradiated
with the excitation light. Hence, it is possible to restrain
fragment ions from being irradiated by the excitation light to be
secondary-dissociated. Although a fragment ion may pass through the
excitation light irradiated space (area to which the excitation
light is irradiated), the transit time is generally short. Hence,
the fragment ion is not easily excited and dissociated.
[0018] However, the reaction rate of a photodissociation varies
according to ion species. Some kinds of generated fragment ions may
be secondary-dissociated before they have been made to resonantly
vibrate to have a vibration amplitude large enough to go out of the
excitation light irradiated space. Hence, the mass spectrometer
according to the first aspect of the present invention may further
include a gas introducer for introducing a predetermined gas into
the ion trap in order to control the reaction rate of the
photodissociation.
[0019] An introduction of a buffer gas into the ion trap during the
photodissociation in this configuration retards the ion's
photodissociation reaction since the internal energy of an ion that
has increased by absorbing a photon for example is taken away by
contacting the buffer gas. As a result, if the excitation light
hits a fragment ion generated by a primary dissociation as
described earlier, since the time period until the secondary
dissociation occurs is elongated, a large vibration amplitude can
be given to the fragment ion to go out from the excitation light
irradiated space before it is secondary-dissociated. Accordingly,
it is possible to further restrain the secondary dissociation of
the fragment ions.
[0020] In the mass spectrometer according to the second aspect of
the present invention, ions to be analyzed are first trapped as
precursor ions in the ion trap. The selection of the precursor ions
may be carried out either inside or outside the ion trap. In each
case, the precursor ions concentratedly exist around the center of
the trapping space of the ion trap. The excitation light emitter
irradiates an excitation light to miss the center of the trapping
space. That is, the excitation light is irradiated to an area
surrounding the center. At the same time, the excitation signal
generator generates an excitation signal having a frequency that
selectively makes the precursor ion resonantly vibrate, and applies
it to at least one of the electrodes that constitute the ion trap.
For example, in the case where the ion trap is a three-dimensional
quadrupole type ion trap with one ring electrode and two end cap
electrodes, the excitation signal may be applied between the both
end cap electrodes.
[0021] Since precursor ions are excited by the effect of an
electric field formed inside the ion trap by the excitation signal,
they do not remain in the center of the trapping space, but go into
the previously described excitation light irradiated space. The
precursor ions are irradiated by the excitation light in that area,
and are dissociated by a photodissociation (primary dissociation)
to generate fragment ions. On the other hand, since the generated
fragment ions are not excited by the effect of the aforementioned
excitation electric field, they are affected by the trapping
electric field and concentratedly gather around the center of the
trapping space. Accordingly, the fragment ions are not easily
irradiated by the excitation light, and it is possible to restrain
the secondary dissociation of fragment ions.
[0022] In a preferable embodiment of the second aspect of the
present invention, the excitation light emitter may irradiate the
excitation light to surround the center of the trapping space of
the ion trap. Specifically, the excitation light irradiated area is
circular-shaped, and the irradiated area may be preferably set so
that the center portion to which the excitation light is not
irradiated is placed in the middle of the trapping space.
[0023] In this configuration, when a selectively excited precursor
ion goes out from the center of the trapping space, it is
irradiated by the excitation light with higher probability.
Accordingly, the precursor ion's dissociation efficiency is
increased.
[0024] In the mass spectrometer according to the second aspect of
the present invention, for example, the ion trap may be a
three-dimensional quadrupole type ion trap with one ring electrode
and two end cap electrodes, or a linear ion trap in which a
plurality of rod electrodes with curved inner surface are aligned
parallel. In each case, the size of the area in which an ion exists
with high probability around the center of the trapping space of
the ion trap is almost determined as a certain portion of the
distance between the electrodes.
[0025] Hence, in order to prevent the fragment ions generated by
the primary dissociation from being irradiated with the excitation
light, it is preferable that the space to which the excitation
light is irradiated by the excitation light emitter be located away
from the center of the trapping space of the ion trap by 2.5% or
above the distance between the two end cap electrodes in the case
where a three-dimensional quadrupole type ion trap is used. In the
case where a linear ion trap is used, it is preferable that the
space to which the excitation light is irradiated by the excitation
light emitter be located away from the center of the trapping space
of the ion trap by 2.5% or above the distance between inner curved
surfaces of two facing rod electrodes.
[0026] With the mass spectrometers according to the first and
second aspects of the present invention, it is possible to restrain
fragment ions, which were generated by photo-dissociation when a
precursor ion is irradiated by an excitation light, from being
further dissociated (secondary dissociation). Accordingly, the
signal intensity of the fragment ions' peaks does not decrease when
an MS/MS spectrum is created. This ensures a high S/N.
[0027] In the mass spectrometers according to the first and second
aspects of the present invention, two or more excitation light
emitters may be provided, and the excitation signal generator may
generate an excitation signal including two or more frequencies
each corresponding to an ion to be resonantly vibrated. The ions to
be resonantly vibrated include fragment ions of different kinds
generated from a precursor ion, or plural precursor ions when a
target ion is accompanied by molecular-related ions such as adduct
ions or ions devoid of water molecule(s).
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic configuration diagram of an ion trap
time-of-flight mass spectrometer according to an embodiment (first
embodiment) of the first aspect of the present invention.
[0029] FIG. 2A is a schematic diagram for spatially illustrating
the relation between the ion trapping space and the excitation
light irradiated space within the ion trap of the mass spectrometer
of the first embodiment.
[0030] FIG. 2B is a diagram illustrating the relation between the
ion trapping space and the excitation light irradiated space on the
ion's existence probability.
[0031] FIG. 3 is a diagram illustrating an example of a frequency
characteristic of an excitation signal in the mass spectrometer of
the first embodiment.
[0032] FIGS. 4A, 4B and 4C illustrate measurement results of the
peak intensity's variation of a precursor ion and four kinds of
fragment ions when an irradiation time of an infrared laser as an
excitation laser is changed.
[0033] FIG. 5 is a schematic configuration diagram of an ion trap
time-of-flight mass spectrometer according to an embodiment (second
embodiment) of the second aspect of the present invention.
[0034] FIG. 6 is a schematic diagram for spatially illustrating the
relation between an ion trapping space and an excitation light
irradiated space within the ion trap of the mass spectrometer of
the second embodiment.
[0035] FIG. 7 is a schematic configuration diagram of an ion trap
of a mass spectrometer according to another embodiment of the
second aspect of the present invention.
[0036] FIG. 8 is a schematic configuration diagram of an ion trap
of a mass spectrometer according to further another embodiment of
the second aspect of the present invention.
[0037] FIG. 9 is a diagram for explaining a signal intensity's
reduction by a secondary dissociation.
EXPLANATION OF THE NUMERALS
[0038] 1 . . . Ion Trap [0039] 11 . . . Ring Electrode [0040] 12,
13 . . . End Cap Electrode [0041] 14 . . . Entrance Aperture [0042]
15 . . . Exit Aperture [0043] 16, 17, 18 . . . Laser Irradiation
Aperture [0044] 2 . . . Ion Source [0045] 20 . . . RF Voltage
Generator [0046] 21, 25 . . . Excitation Signal Generator [0047]
22, 26 . . . Excitation Laser Emission Source [0048] 23 . . . Gas
Introducer [0049] 24 . . . Controller [0050] 3 . . . Time-Of-Flight
Mass Spectrometer [0051] 4 . . . Flight Space [0052] 5 . . . Ion
Detector
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0053] Embodiments of a mass spectrometer according to the present
invention will be explained hereinafter with reference to
figures.
First Embodiment
[0054] FIG. 1 is a schematic configuration diagram of an ion trap
time-of-flight mass spectrometer (IT-TOFMS) according to an
embodiment (the first embodiment) of the first aspect of the
present invention.
[0055] Inside an evacuated vacuum chamber (not shown), a
three-dimensional quadrupole type ion trap 1 is disposed. The ion
trap 1 is composed of a ring electrode 11 and a pair of end cap
electrodes 12, 13 opposing each other (right and left in FIG. 1)
with the ring electrode therebetween. The inner surface of the ring
electrode is formed hyperboloid-of-one-sheet-of-revolution and the
inner surface of the end cap electrodes are formed
hyperboloid-of-two-sheets-of-revolution. These electrodes 11, 12,
and 13 form a trapping space A for trapping ions by a trapping
electric field in the space surrounded thereby.
[0056] Outside an entrance aperture 14 bored through the
entrance-side end cap electrode 12, an ion source 2 such as a MALDI
for example is placed. On the other hand, outside an exit aperture
15 bored thorough the exit-side end cap electrode 13, a
time-of-flight mass spectrometer 3 including a flight space 4 for
separating ions according to their mass-to-charge ratios and an ion
detector 5 are placed. The ion source 2 is not limited to a MALDI,
and various types of known ion sources may be used in its place.
The time-of-flight mass spectrometer 3 may be replaced with one of
the other types of mass spectrometers. Alternatively, it is
possible to place only an ion detector outside of the exit aperture
15, regarding the ion trap 1 itself as a mass spectrometer.
[0057] In the center of the ring electrode 11 along the axis
(R-axis) of the ring electrode 11, a laser irradiation aperture 16
is bored. A laser light as an excitation light emitted by the
excitation laser emission source 22 flies through the laser
irradiation aperture 16 to the core (center of the trapping space
A) of the ion trap 1. Accordingly, the laser light irradiates
(flies through) the core of the ion trap 1. A gas introducer 23
provides a buffer gas to the inside of the ion trap 1.
[0058] An RF voltage generator 20 is connected to the ring
electrode 11, and an excitation signal generator 21 is connected to
both of the end cap electrodes 12, 13. The RF voltage generator 20
and the excitation signal generator 21 are respectively controlled
by a control signal provided by a controller 24 to generate an
alternating voltage having a predetermined frequency and
predetermined amplitude. It is possible to superpose a direct
current voltage to the alternating voltages according to necessity.
The controller 24 includes a CPU, RAM, and other components, and
controls the RF voltage generator 20 and the excitation signal
generator 21 based on a preset control program. The controller 24
also controls the operation of the ion source 2, the excitation
laser emission source 22, the gas introducer 23, and other
components.
[0059] The operation for obtaining an MS/MS spectrum of an ion
having a specific mass with this IT-TOFMS will be explained.
[0060] First, a buffer gas such as He is introduced in a pulsed
manner from the gas introducer 23 to fill the ion trap 1 under the
control of the controller 24. Then, a predetermined RF voltage is
applied from the RF voltage generator 20 to the ring electrode 11
to form a quadrupole type electric field for trapping ions. From
this state, when various ions generated from a sample to be
analyzed in the ion source 2 are introduced into the ion trap 1
through the entrance aperture 14, the ions collide with the buffer
gas and lose their kinetic energy, or "cooled". The ions are
eventually trapped in the quadrupole type electric field and gather
around the center of the trapping space A.
[0061] After such various kinds of ions are trapped in the trapping
space A within the ion trap 1, an excitation signal for making ions
other than precursor ions vibrate wildly is generated by the
excitation signal generator 21, and is applied to the end cap
electrodes 12 and 13 in order to make only precursor ions remain
within the ion trap 1. Undesired ions other than the precursor ions
to be targeted are consequently dispersed to the outside of the ion
trap 1 via the entrance aperture 14 and the exit aperture 15.
[0062] After the precursor ions are selected as just described, the
excitation laser emission source 22 is driven, so as to
photo-dissociate the selected precursor ions, to irradiate an
excitation laser light to the center of the trapping space A (an
excitation light irradiated space B) within the ion trap 1. At the
same time, an excitation signal having a frequency that does not
make the precursor ions vibrate but makes fragment ions vibrate
that were generated in the primary dissociation of the precursor
ions is generated in the excitation signal generator 21, and is
applied to the end cap electrodes 12 and 13. The details of the
excitation signal will be specifically explained later.
[0063] FIG. 2A is a schematic diagram for spatially illustrating
the relation between an ion trapping space A and an excitation
light irradiated space B within the ion trap of the mass
spectrometer of the first embodiment. FIG. 2B is a diagram
illustrating the relation between the ion trapping space A and the
excitation light irradiated space B on the ion's existence
probability. As illustrated in FIG. 2B, most of the ions that are
not excited exist within the trapping space A and they especially
exist in the center portion the trapping space A with high
probability. Since an excitation laser light is irradiated to this
space, the laser light efficiently hits the precursor ions that are
not excited, and their photodissociation is accelerated. This makes
the precursor ions dissociated to generate fragment ions with a
smaller mass.
[0064] Although the fragment ions are also trapped by the trapping
electric field, they are in addition affected by the electric field
formed by the excitation signal and begin to vibrate wildly in the
Z-direction. Hence, as illustrated in FIG. 2A, the fragment ions
are off the excitation light irradiated space B for a long time.
Therefore, they are not easily secondary-dissociated by a
photodissociation. That is, it is possible to photo-dissociate
precursor ions with high probability, and to restrain the primary
fragment ions generated by the photodissociation from being
photo-dissociated.
[0065] Since the photodissociation of precursor ions occurs within
the excitation light irradiated space, however, if it takes too
much time to make generated fragment ions vibrate with an amplitude
significant enough to be out of the excitation light irradiated
space, a secondary dissociation may take place. The reaction rate
of a photodissociation varies according to ionic species; a
fragment ion having a high reaction rate in particular is more
likely to be secondary dissociated. Hence, it is preferable to
introduce a small amount of buffer gas into the ion trap 1 when the
laser light is emitted in order to deliberately restrain the
fragment ion's secondary dissociation, although it is preferable in
general to keep the inside of the ion trap 1 in a high-vacuum state
when a photodissociation is accelerated by irradiating an
excitation laser light.
[0066] If a buffer gas is introduced into the ion trap 1, the
reaction rate of a photodissociation decreases because ions that
have absorbed photons gain the internal energy and become more
likely to collide with the buffer gas to be relaxed from the
excitation state. Hence, although the efficiency of the primary
dissociation of precursor ions themselves decreases, the secondary
dissociation of the primary fragment ions is further restrained as
well. Although such effects are put out in an ultraviolet light
dissociation as well, it is more prominent and effective in
Infrared Multiphoton Dissociation in particular.
[0067] After the precursor ions are dissociated by a
photodissociation for a predetermined period of time as described
earlier, a voltage capable of evacuating the ions trapped within
the ion trap 1 is applied to the end cap electrodes 12 and 13. An
initial kinetic energy is accordingly given to the fragment ions
and they are collectively emitted from the exit aperture 15 and
introduced into the time-of-flight mass spectrometer 3 to be
mass-analyzed. Then, an MS/MS spectrum is created by processing the
detection signals from the ion detector 5 by a data processor (not
shown).
[0068] Examples of an excitation signal to be applied to the end
cap electrodes 12 and 13 on a photodissociation will be explained.
In the case where the mass of the fragment ions generated by a
precursor ion's dissociation is known in advance, a sine wave
signal having a frequency that corresponds to the resonance
frequency of the fragment ions can be used as an excitation signal
in order to selectively make only the fragment ions vibrate. For
example, when only one kind of fragment ion is included, a sine
wave signal (or a rectangular wave signal or the like) having a
single frequency can be used as an excitation signal as illustrated
in FIG. 3A. When plural kinds of fragment ions are included,
different sine wave signals, each signal having a single frequency,
may be synthesized to be used as an excitation signal.
[0069] In the case where the mass of the fragment ions are unknown
or the number of masses are many, a broadband signal without the
frequency corresponding to the resonant frequency (in practice,
without a predetermined width of frequencies around the resonant
frequency) of a precursor ion may be preferably used as an
excitation signal. Such broadband signal may be generated by using
a known method, for example, disclosed in Japanese Patent No.
3470671.
[0070] Next, an experimental result for verifying the effect of the
mass spectrometer according to the first aspect of the present
invention will be explained.
[0071] In this experiment, reserpine (molecular weight: 608) was
used as a sample, and electrospray ionization (ESI) ion source was
used as an ion source. Proton-added ions (m/z 609) generated by
this ion source were left as precursor ions within an ion trap.
Then an infrared laser light as an excitation laser light was
irradiated to make them infrared-multiphoton dissociated. It is
known that fragment ions with mass-to-charge ratio m/z of 236, 397,
and 448 are generated when reserpine is dissociated by a collision
induced dissociation. It is also known that a peak of m/z 363 is
observed in addition other than the four peaks after an infrared
multiphoton dissociation.
[0072] FIG. 4 illustrates the measurement results of the peak
intensity's variation of a precursor ion (m/z 609) and four kinds
of fragment ions (m/z 236, 363, 397, and 448) when the irradiation
time of an infrared laser as an excitation laser was changed. FIG.
4A is a result in the case where no excitation signal was applied
when an infrared-multiphoton dissociation was taking place. That
is, FIG. 4A is a result of a conventional method. The longer the
laser irradiation time became, the higher the internal energy of
the precursor ions accordingly became and the weaker the peak
intensity became since an infrared-multiphoton dissociation began
to take place. In contrast to the decrease, the peak intensity of
the fragment ions (m/z 236, 397, and 448) generated in the primary
dissociation increased in a range where the laser irradiation time
was shorter than a certain time. However, if the laser irradiation
time became longer than that previously mentioned, the peak
intensity of primary fragment ions decreased based on an effect of
a secondary dissociation. After the peak intensity of other
fragment ions shifted to a decreasing rate, the peak intensity of a
fragment ion of m/z 363 began to increase as if replacing them.
Therefore, it is possible to presume that this was a secondary
fragment ion generated by a secondary dissociation.
[0073] It is understood that, in this example, approximately 8 ms
of laser irradiation time is necessary to maximize the peak
intensity of the fragment ions by a primary dissociation. In this
case, however, half of the precursor ions still remained
undissociated, and the dissociation efficiency was not very
high.
[0074] FIG. 4B is a result obtained in the case where a sine wave
signal of frequency 74 kHz as an excitation signal was applied
between the end cap electrodes so that fragment ions of
mass-to-charge ratio of m/z 448 were selectively resonantly
vibrated. An infrared laser light was irradiated at the same time.
The peak intensity's change of the fragment ions (m/z 236, 397)
other than m/z 448 can be regarded a fluctuation of a signal
intensity, and was as much as that of FIG. 4A. In contrast, since a
fragment ion of m/z 488 was selectively excited to be out of the
infrared irradiated space, it was barely affected by a secondary
dissociation. Therefore, the peak intensity increased almost
monotonically as the laser irradiation time became longer. In the
case where the laser irradiation time was longer than 10 ms, the
peak intensity was saturated. Therefore, the peak intensity's
decrease by a secondary dissociation did not occur as for the
fragment ion of m/z 448.
[0075] FIG. 4C is a result obtained in the case where a sine wave
signal of frequency 149 kHz as an excitation signal was applied
between the end cap electrodes so that fragment ions of
mass-to-charge ratio of m/z 236 were selectively resonantly
vibrated. In this case, only the fragment ion of m/z 236 was not
affected by a secondary dissociation, and its signal intensity
monotonically increased as the laser irradiation time became
longer.
[0076] The result indicates that the fragment ions selectively
vibrated while an infrared multiphoton dissociation is occurring
are not affected by a secondary dissociation and a high peak
intensity can be therefore assured. In this experiment, a sine wave
signal with a single frequency was applied as an excitation signal
to the end cap electrodes to restrain a predetermined fragment
ion's secondary dissociation, in order to clearly show the
fundamental effect of the present invention. However, in order to
increase the signal intensity by restraining the secondary
dissociation's affect as for plural or many fragment ions, a
broadband signal as described earlier (synthetic waveform of
discrete frequencies across a broadband without a resonant
frequency of a precursor ion) may be applied to the end cap
electrodes as an excitation signal as a matter of course.
Second Embodiment
[0077] FIG. 5 is a schematic configuration diagram of an ion trap
time-of-flight mass spectrometer (IT-TOFMS) according to an
embodiment (the second embodiment) of the second aspect of the
present invention. In FIG. 5, like elements are denoted by like
numerals as in the first embodiment which was described
earlier.
[0078] One of the essential differences between the second
embodiment and the first is that the excitation laser light is not
irradiated to the center of the trapping space A of the ion trap 1,
but is purposely irradiated to the area off the center. For this
purpose, the laser irradiation aperture 17 is placed off the center
axis of the ring electrode 11. In addition, the excitation signal
generator 25 applies an excitation signal having a frequency that
selectively makes a precursor ion to be targeted vibrate (but not
making a fragment ion vibrate) to the end cap electrodes 12 and 13
when making a photodissociation occur by irradiating a laser light.
In this case, the excitation signal can be generated easier than
the first embodiment since a sine wave signal with a single
frequency or a rectangular wave signal will do.
[0079] FIG. 6 is a schematic diagram for spatially illustrating the
relation between the ion trapping space A and the excitation light
irradiated space B within the ion trap. When the excitation signal
is applied to the end cap electrodes 12 and 13, the precursor ions
vibrate wildly in the Z-axis direction by the effect of the
excitation electric field formed within the ion trap 1. If no
excitation signal is applied, precursor ions do not enter the
excitation light irradiated space B; if the precursor ions are
excited, they pass through the excitation light irradiated space B,
then absorb photons during the crossing, and are presently
photo-dissociated. This generates fragment ions, and such fragment
ions are not affected by the excitation electric field and do not
vibrate wildly although they are affected by the capturing electric
field by the RF voltage applied to the ring electrode 11. The
fragment ions consequently gather around the center of the ion
trapping space A. That is, the fragment ions are not easily
secondary dissociated since they are out of the excitation light
irradiated space B and are not irradiated by the excitation laser
light. Therefore, the amount of the primary fragment ions increases
as the laser irradiation time becomes longer, which enhances the
S/N of an MS/MS spectrum.
[0080] According to a simulated calculation by the inventors of the
present invention, when the distance between the two end cap
electrodes 12 and 13 was 20 mm, the spread width of the ion cloud
that was sufficiently cooled by a collision with a buffer gas was
under .+-.0.5 mm from the center of the ion trap 1. Hence, if the
irradiated area by an excitation laser is away from the center area
of the ion trap 1 by 0.5 mm or greater, the secondary dissociation
which occurs when an excitation laser light hits fragment ions can
be efficiently avoided. Since it is possible to consider that the
same model is established with different sizes of the ion trap 1,
if the excitation light irradiated space B is off the center of the
ion trap 1 by 2.5% or above the distance between the end cap
electrodes, the effect of restraining secondary dissociation is
substantially exerted.
[0081] With the configuration illustrated in FIGS. 5 and 6,
however, significantly elongating the time period while the
precursor ions that vibrate with large amplitude is difficult.
Hence, it is necessary to elongating the laser irradiation time in
order to enhance the dissociation efficiency. Then, it is possible
to modify the configuration of the ion trap 1 as illustrated in
FIG. 7 so as to ensure that the precursor ions vibrated remain in
the excitation light irradiated space B for a longer period of
time. With this configuration, the excitation laser emission source
26 emits a laser light whose sectional form of the irradiated space
has a circular shape. The laser light flies through the laser
irradiation aperture 18 which is assuredly and cylindrically placed
in the ring electrode 11 for example and forms the excitation light
irradiated space B whose center portion is an unirradiated space
and the surrounding area of the unirradiated space is an irradiated
space. Such a laser light with a specific form can be generated,
for example, by using a method disclosed in Japanese Unexamined
Utility Model Application Publication No. 62-47959.
[0082] Since the excitation light irradiated space B is larger in
this configuration, chances are high for precursor ions that
vibrate by the excitation electric field to be irradiated by the
excitation laser light. Hence, the precursor ion's dissociation
efficiency increases as much. On the other hand, the fragment ions
gathering around the center of the trapping space A are not
irradiated by the excitation laser light. The secondary
dissociation can be therefore prevented.
[0083] Although the excitation laser light was irradiated through
the laser irradiation apertures 16, 17, and 18 which were bored
through the ring electrode 11 in the aforementioned embodiment, the
excitation laser light can be slantly irradiated through the gap
between the ring electrode 11 and the end cap electrode 12 (or 13)
as illustrated in FIG. 8. In this case, the configuration is simple
since there is no need for placing a laser irradiation aperture in
the ring electrode 1. Moreover, the ions' trapping efficiency can
be enhanced since the disarrangement of the trapping space electric
field based on the placement of a laser irradiation aperture in the
ring electrode 11 dos not occur.
[0084] The embodiment described thus far is merely an embodiment of
the present invention, and may be modified or changed within the
scope of the present invention. For example, although a
three-dimensional quadrupole type ion trap was used in the
embodiments described earlier, a linear ion trap in which four (or
more) rod electrodes whose inner surface is hyperboloidal or
cylindrical are aligned parallel and a trapping space is formed in
a space surrounded by the rod electrodes can also be used in the
present invention.
* * * * *